Best cooling tower for power plants
Lexicon> letter K> cooling tower
Definition: a system for releasing waste heat into the environment
English: cooling tower
Categories: electrical energy, engines and power plants, heating and cooling
Author: Dr. Rüdiger Paschotta
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Original creation: 06/29/2010; last change: 03/14/2020
A Cooling tower is a system whose function is to release large amounts of waste heat into the environment. Thermal power plants in particular usually have a cooling tower to dissipate the unavoidable waste heat from the heat engine (usually a steam turbine) if it cannot be used (e.g. as district heating). Even if most of the heat is used (→ combined heat and power), a cooling tower can dissipate the excess heat under special circumstances (e.g. reduced heat demand in summer). Under certain circumstances, a cooling tower is then also used continuously to increase the electrical efficiency by reducing the pressure in the condenser.
Even if river water is available for cooling at a power plant, a cooling tower is usually used to release a large part of the waste heat into the ambient air. This avoids excessive heat input into the river, which would otherwise have negative effects, e.g. B. could have on the fish. The water to be cooled in the cooling circuit then first flows through the cooling tower and is only then further cooled by a heat exchanger with river water in order to enable the power plant to be more efficient. It may be surprising that a stronger dissipation of waste heat increases the efficiency, although this heat energy is lost. B. a steam turbine more efficient when the cooling is effective.Depending on the type of power plant, the proportion of energy released via the cooling tower can be significantly lower or also much higher than the amount of electrical energy obtained.
The proportion of the primary energy used in the power plant that ultimately has to be dissipated via the cooling tower depends heavily on the type of power plant. In modern coal-fired power plants, the efficiency is z. B. 45%, so that 55% of the energy is generated as waste heat, most of which is given off via the cooling tower. In the case of nuclear power plants, the efficiency is usually significantly lower, e.g. B. 35%, so that 65% is generated as waste heat: The amount of waste heat is almost twice as high as the amount of electrical energy generated. With the same electrical output, the amount of waste heat is around half that of a modern coal-fired power plant. Only high-temperature reactors, which have so far been used very rarely, allow significantly higher efficiencies. The situation is even more extreme with geothermal power plants, where the electrical efficiency is very low due to the low steam temperatures - often in the order of 10%. If combined heat and power is practiced, the energy loss in the cooling tower can of course be correspondingly lower.
The climate-damaging fossil fuel-fired power plants arises mainly from carbon dioxide emissions and not from the release of heat via cooling towers. However, a use of the waste heat z. B. enable reduced emissions for heating.
Functional principles and designs of cooling towers
The basic principle of a cooling tower is that heat is transferred from the warm cooling water to the cooler ambient air. Because of the low heat capacity of air, huge amounts of air have to be moved through a power plant cooling tower. This is usually done passively via the Chimney effect: The heated air in the cooling tower expands, so it loses its density, thus experiences a lift and rises upwards. From below (at the foot of the Natural draft cooling tower) fresh air is drawn in.
A sufficiently strong chimney effect requires a certain height of the cooling tower. Much smaller designs are possible by additionally driving the air with powerful fans. However, this requires an additional expenditure of energy, which affects the overall efficiency of the power plant.The evaporation of water can significantly increase the effectiveness of a cooling tower, but unfortunately leads to the formation of vapor plumes.
The effectiveness of a cooling tower can be significantly increased by not only heating air, but also by evaporating water. In such a Wet cooling tower the water to be cooled is sprayed so that, on the one hand, there is good thermal contact with the air and, on the other hand, a smaller part (a few percent) of the circulated water evaporates. The evaporation creates large amounts of Evaporation cooling: It leads to heat dissipation as latent heat. Above the cooling tower, part of the water vapor generated condenses again, creating the familiar vapor plumes which, due to the increased temperature, can rise to great heights (often up to the cloud cover).With wet cooling towers there is a risk of colonization with microorganisms, which is why chemicals often have to be used.
Wet cooling towers are particularly effective; H. they can dissipate large amounts of heat and bring the cooling water to relatively low temperatures. Disadvantages are the high water consumption and the formation of steam plumes. In addition, with circulation cooling (use of the water in a circuit), measures against algae growth, calcification and colonization with legionella bacteria are necessary; chemicals are usually used here. There is a risk of freezing in particularly cold locations. All of this will come with Dry cooling towers avoided in which the cooling water does not come into contact with the air; it just flows through a tubular network with cooling fins that release the heat into the air. However, dry cooling towers are far less effective and thus impair power plant efficiency and / or have to be built even larger.
A mixed solution is that Hybrid coolingwhich also evaporates water, but in smaller quantities. Here, strong fans are required, with which a warm air stream is mixed with the steam, so that the exhaust air is less humid and accordingly less strong steam plumes.
In some cases, a cooling tower doubles as a chimney. The exhaust gas is released into the cooling tower, and the strong buoyancy of the cooling tower also serves to transport the exhaust gas to great heights. This is particularly useful if the flue gas temperature is relatively low after the flue gas scrubbing, so that an ordinary chimney would not provide enough lift.
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See also: power plant, steam turbine, waste heat
as well as other articles in the categories of electrical energy, engines and power plants, heating and cooling
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